1. Field of the Invention
The present invention relates generally to chemical or biochemical detection and more specifically to molecular or ion detection by selective thermal heating with incident infrared light resonant with one or more selected infrared absorption bands, and examination of vapor or light produced as a result of said incident light.
2. Description of the Prior Art
Low vapor pressure analytes, in their condensed physical state, such as explosives, drugs of abuse, and chemical warfare agents, are difficult to detect in a non contact or stand off mode, using conventional technology. Since the handling of these materials leaves persistent trace contaminations on contacted surfaces (e.g., door handles, pockets, hands, containers, etc.), manual swipes are often used to lift particles or residue of the analytes for subsequent analysis, for example in airport hand luggage screening. However, performing a manual swipe of each potential suspect and container is often impractical or inconvenient, and it inhibits covert detection.
A summary of existing and emerging technologies for the portable detection of contaminants, such as chemical warfare agents, is outlined by Michael W. P. Petryk in his article entitled “Promising Spectroscopic Techniques for the Portable Detection of Condensed-Phase Contaminants on Surfaces,” found in Applied Spectroscopy Reviews, 42: 287-343, 2007, the entire contents of which are incorporated herein by reference. Moreover, U.S. Pat. No. 6,998,156 to Bubb et al. describes using an infrared laser to vaporize target material. The entire contents of the Bubb patent are incorporated herein by reference.
For trace detection of explosives or drugs, the current technologies typically work well as long as particles can be collected by some physical means and then thermally converted into vapor for analysis or detection. The sampling techniques use non-selective removal of particles from a selected few surfaces which may have been contaminated with particles of explosives and transfer them onto a heated surface which is interfaced to an ion mobility spectrometer (IMS) or other explosive detection system (EDS). Previously, this has required either a physical rubbing process or forced air removal, neither of which is material selective or practical for any significant stand off distance and can add a significant time and personnel cost burden to the detection process. In addition, the efficiency of physical removal of particles from a surface as particles depends on the techniques used, the training level of the person removing the particles, and the rubbing or contaminated surface material or surface roughness of contaminated surfaces.
Known methods of heating trace samples of explosives for detection purposes include broad band IR sources which heat in a non selective fashion. This approach consumes much more power than a selective heating process and generally heats everything incident with the heating source. This increases the general background level in the vapor phase of all the volatile chemicals in the material examined and can result in an increase in signal clutter or false alarms, especially when the substrate materials or additional contaminants being examined are of a complex natural origin such as leather, wood or food products.
Laser induced breakdown spectroscopy (LIBS) is an alternative laser based technique but this requires significantly higher power and results in the destruction of the sample of interest and the surface on which it directly resides. The lasers used for LIBS are typically high power (10 mJ or greater) with short wavelengths (UV to near IR) and are not considered safe for environments where humans might be exposed or for the integrity of the substrate being examined. LIBS is a type of atomic emission spectroscopy which utilizes a highly energetic laser pulse as the excitation source to ablate material, reducing it to its elemental constituents. LIBS can analyze any matter regardless of its physical state, be it solid, liquid or gas. Because LIBS detects elements, its selectivity in the presence of many materials is suspect and is reliant on signal ratios of elements which can be confused when mixtures of materials are present. Nitrogen, for example, is present in many explosives but it is also prevalent in cotton or wool fiber or any proteinacous material. Trace explosives present on natural fibers would be difficult to detect accurately with LIBS.
Raman spectroscopy is an emerging standard for optical identification and characterization of known and unknown samples. It couples to signature vibrational modes of the analyte and is complementary to infrared spectroscopy. Its main drawback is in its inefficiency because typically only one photon is Raman scattered for every million photons incident on the sample. Furthermore, Raman is isotropic, meaning there is no preferred direction for the scattered light to travel. This limits its application for stand off detection. For a fixed collection optic diameter, the photon collection efficiency decreases proportional to the second power of the distance to the sample under interrogation. Finally, Raman efficiency is optimized with high photon energy light which is not eye-safe to use in the presence of people.
Photo-thermal spectroscopy is another potential tool that is used in stand-off detection. In this technique, the sample is heated with a non-resonant, not eye-safe laser (usually visible wavelength of near-IR) in a periodic fashion (using a mechanical chopper). The detected signal consists of the amplitude of the heated signal measured by an IR detector (or some other means) and its phase-angle shift with respect to laser heating. This method differs from the present invention, in part, because it does not take advantage of the resonant nature of absorption of IR radiation which allows analyte selectivity right at the excitation stage and with much less laser power to achieve suitable heating.
One method of detecting explosives uses a broadband heating source connected to an IMS. One problem with this method is that the entire composition of the surface, and possibly deeper, is heated which makes accurate detection of the analyte more difficult. Another method of detecting explosives, narcotics and other chemical substances, uses a laser source to ablate the particles, then collects them and subsequently analyzes them. Unfortunately, the ablation process may damage the analyte, resulting in additional signal clutter and possible reduction in the principle analyte signal, and this method requires a separate collection step.